It is known to add ancillary signals to radio frequency signals in order either to monitor the broadcasts of programs or to measure audiences of programs. Such programs may include television programs, radio programs, and/or the like, and the broadcast of such programs may include the transmission of these programs over the air, over a cable, via a satellite, and/or the like.
When monitoring the broadcast of programs, a monitoring system typically determines the identity of the programs which were broadcast, the regions in which these programs were broadcast, the times at which these programs were broadcast, and the channels over which these programs were broadcast. One monitoring system, which is commonly used within the United States, is referred to as the "AMOL" system and is taught by Haselwood, et al. in U.S. Pat. No. 4,025,851. This "AMOL" system adds a source identification code to selected horizontal lines on the vertical blanking intervals of the program signal. Monitoring equipment, which is located in selected regions throughout the United States, determines the identity of the programs which are broadcast by detecting the source identification codes of the broadcast programs. The monitoring equipment stores, for later retrieval, these detected source identification codes together with the times at which these source identification codes were detected and the channels on which these source identification codes were detected.
When measuring the audiences of programs, an audience measurement system typically determines which programs were viewed by which members of a statistically sampled panel of dwellings. One type of an audience measurement system uses an ancillary signal to monitor a program receiver, and commonly injects this ancillary signal (which may be a simple tone, or a complex digital code) into a channel that may be carrying a viewed program. The ancillary signal is injected by an apparatus which is located at or near the program receiver being monitored. The detection of such an ancillary signal provides an indication that the program receiver is on and is tuned to the channel carrying the detected ancillary signal. Signal injection systems are taught, inter alia, in PCT/US93/09458 by Mostafa, et al.
Adding an ancillary signal to a program signal must generally be done so as to minimize the probability that the ancillary signal will interfere with any aspect of the basic program signal. Several systems are known which add ancillary signals at selected frequencies within the radio frequency band, or channel, of a program signal. For example, in U.S. Pat. No. 2,982,813, Hathaway teaches a system which frequency interleaves the ancillary and program signals. In U.S. Pat. No. 3,838,444, Loughlin, et al. teach a system which compatibly adds and transmits an ancillary signal in a low energy density portion of a color television frequency spectrum. The low energy density portion of interest is located between the video carrier and the color subcarrier of the NTSC television signal, and is at a frequency which is about 2.4 MHz above the video carrier peak in the radio frequency band of the NTSC television signal. The teachings of Loughlin, et al. in U.S. Pat. No. 3,838,444 are herein incorporated by reference.
Furthermore, in U.S. Pat. No. 3,842,196, Loughlin discloses an improved system which minimizes interference between a primary ancillary signal and the program signal by adding a redundant ancillary signal. The redundant ancillary signal is transmitted with an inverted polarity as compared to the primary ancillary signal so that artifacts which may be caused by the addition of an ancillary signal to the program signal are canceled in the video display.
However, the accuracy with which these systems set the absolute frequency of the ancillary signal which is to be injected into the program signal is limited. For example, a reasonable error (e.g., .+-.50 KHz) in setting the frequency (e.g., 100 MHz) of the ancillary signal can cause the injected ancillary signal to overlap a harmonic of the horizontal scanning frequency. If the frequency of the ancillary signal, because of an error, overlaps a harmonic of the horizontal scanning frequency, the ancillary signal will interfere with the program signal. This interference is annoyingly apparent to a viewer. Moreover, if the frequency of the ancillary signal is uncertain, a monitoring system or an audience measurement system may have difficulty in picking up and using the ancillary signal for program monitoring and/or audience measurement purposes.
The present invention overcomes one or more of these problems